Various machines are provided with sliding members that relatively move while being slidably in contact with each other. In a system having such sliding members (referred to as a “sliding system” in the present description, e.g., a sliding machine), the resistance force (sliding resistance) acting on the sliding portions may be reduced thereby to enhance the performance and reduce the energy necessary for operation. Such reduction of sliding resistance is ordinarily achieved by reducing the friction coefficient acting between sliding surfaces.
Friction coefficient acting between sliding surfaces differs depending on the surface condition of each sliding surface and the lubrication state between the sliding surfaces. To reduce the friction coefficient, therefore, it may be contemplated to modify the sliding surfaces and improve the lubricant (lubricant oil) which is supplied between the sliding surfaces. Various types of surface modification for sliding surfaces are known, among which modification has often been performed such that the sliding surfaces are each formed with an amorphous carbon film (so-called diamond-like carbon (DLC) film) that can reduce the friction and has excellent wear resistance. The lubricant is also improved in various manners in accordance with the type of sliding machine, the environment of usage, and other factors, but in many cases the improvement may be achieved by compounding an additive that has an effect of reducing the friction. Descriptions relevant to the above are found in Patent Literature (PTL) below, for example.